1. Field of the Invention
The present invention is generally related to the manufacture of self-supporting structures made of thermoplastic material and, more particularly, to a process by which thermoplastic material can be sprayed into a mold structure to form varying thicknesses at preselected locations of the structure to provide self-supporting structural integrity for the completed component.
2. Description of the Prior Art
At the present time, numerous self-supporting structures, such as boat hulls and the like, are made by spraying a thermoset material into a mold which is shaped to form a desired configuration of the finished product. Generally, two liquids are mixed together in known processes and an exothermic reaction occurs. For example, a liquid resin and a liquid hardener can be mixed and sprayed into the mold. In certain applications, glass strands are added to the mixture prior to the spraying process. The glass strands reinforce the structure and provide the necessary strength for the structure to be self-supporting and have the durability necessary for certain applications, such as in the manufacture of boat hulls.
The known methods of manufacturing self-supporting structures have certain serious disadvantages. First, the cure cycle for many thermoset materials can require as much as twelve hours. In addition, the resin has a solvent within it to lower its viscosity. This solvent typically contains styrenes which have been determined to be harmful to the environment. It is likely that the use of styrenes will be strictly controlled in the future because of these environmental considerations.
As discussed above, the cure cycle of a self-supporting structure can often take as many as twelve hours to complete. If the thermoset material is sprayed in consecutive layers, which is a typical way to manufacture a boat hull by this known process, each layer can require up to forty minutes curing time. In addition, each layer must generally be cured before the application of subsequent layers.
A serious disadvantage of using thermoset material is that the cured material is not suitable for recycling. In other words, the thermoset material, such as polyester resin, can not be melted when the structure is salvaged to permit its use in another product.
It would therefore be significantly advantageous to the manufacture of structural devices, such as boat hulls, if a process could be developed which uses a thermoplastic material instead of a thermoset material. It would also be beneficial if the process could avoid the use of styrenes and other environmentally harmful elements. Additionally, it would be beneficial if the process could use a material which has a reduced cure time, as compared to the thermoset process, and which permits recycling of the formed structures after their useful life is complete.
Thermoplastic material has been used as a protective coating for various components. As a example, Applied Polymer Systems, Inc. performs coating with a process that sprays a polymer coating on the object to be protected. The coating can be used for acid resistance or heat resistance and can comprise polyvinylidenefluoride (PVDF), linear polyethylene (LPE), polypropylene (PP), polypetrafluoroethylene copolymers (PTFE), flexible nylon, copolyamide nylon, polyesters, or any other suitable material. A document titled "Can Develop a Solution to Your Coating Problems" by Applied Polymer Systems, Inc. describes the basic technology used in the process and discusses several applications for the coating process.
U.S. Pat. No. 5,106,910, which issued to Weidman et al on Apr. 21, 1992, discloses a high concentration acid resistant coating. The improved polyamide-based compositions described in this patent are particularly formulated for plasma-spray application to surfaces that are subjected to acidic conditions. The compositions include respective amounts of polyvinylidene fluoride, a polyamide (e.g. N11) and a compatible adhesive such as a nylon perpolymer adhesive.